Several processes for pulp making utilize liquid streams with a high content of caustic compounds to dissolve lignin and free the cellulose fibers of wood. Not only are sodium sulfide and sodium hydroxide useful for dissolving lignin, but any analogous potassium compounds available in the pulping cycle may act in the same manner. However, the potassium compounds become a problem when they pass into the recovery cycle in the spent liquor stream.
Consider the pulping and recovery cycles at the exit from the chemical recovery furnace. Green liquor is formed by dissolving the molten smelt discharged from the furnace into the water of the dissolving tank. Green liquor is causticized to form the working white liquor. In the digester, wood chips are heated under pressure in the white liquor to dissolve the lignin. The weak black liquor from the digester has its water content decreased by evaporation. The resulting heavy black liquor is then conducted to the chemical recovery furnace.
Potassium compounds from the wood are inadvertently added to the weak black liquor in the digester during the process of dissolving the lignin. Recovery of chemical values from mill waste streams added to the black liquor, and the impurities contained in the makeup materials, also add potassium compounds to the black liquor. Therefore, the potassium content of the heavy black liquor stream fed to the chemical recovery furnace comes from several sources. In any event, the potassium compounds in the heavy black liquor continue to build up in the recovery system and create the problems in the chemical recovery furnace which are solved by the present invention.
The heavy black liquor is fed into the chemical recovery furnace where most of the water content of the liquor is flashed into steam, the organic content is pyrolized and burned, and the inorganics are formed into a molten smelt. With the inorganics reduced to a molten smelt, the final step is the fragmenting of this smelt with steam to further the dissolving of the inorganics in an aqueous phase to form green liquor. As previously stated, this green liquor is causticized, its alkali content raised, to form the working white liquor for the digester.
Again, the problem faced by the present invention is created by the potassium content of the heavy black liquor fed into the chemical recovery furnace. It is the practice to form a spray of the black liquor over the burning bed of char so that the water of the heavy black liquor will be largely flashed into vapor and be ejected from the furnace. The temperature of the bed is expected to be maintained at 1600.degree.-1800.degree. F. However, potassium compounds lower the melting point of ash in the bed which, in turn, lowers the bed burning temperature substantially. Unfortunately, this lower bed temperature in the reducing atmosphere maintained in the furnace breaks down the sodium sulfide to form hydrogen sulfide which is burned to sulfur dioxide and, thus, is lost up the furnace stack along with the other vapors of combustion. In addition, the lower melting smelt contaminated by potassium compounds is much more corrosive to the metalic furnace tubes. Therefore, what is needed is an additive to the heavy black liquor stream, on its way to the chemical furnace, which will form a high melting, insoluble reaction product with the potassium to prevent the lowering of the furnace bed and smelt-melting temperatures.